Field emission electron sources, often referred to as field emission materials or field emitters, can be used in a variety of electronic applications, e.g., vacuum electronic devices, flat panel computers, television displays, emission gate amplifiers, klystrons, vacuum microelectronics application, and the like.
Field emission displays (FED) are a promising challenge to the liquid crystal displays (LCD) currently used for most flat panel electronic displays. Compared to LCD's, FED's are three times brighter, half as thick, have almost twice the viewing angle, one fourth the power consumption and more than twice the thermal operating range. Field emitters are in their incipient stages of development and as such there are many problems associated therewith.
Some contemporary field emitters have etched silicon or silicon microtips; however, these require expensive and elaborate fabrication techniques. Additionally, such field emission cathodes suffer from relatively short lifetimes due to erosion of the emission surface from positive ion bombardment.
Contemporary field emitters with sharp miniature tips made of metals or semiconductors have reasonably good properties necessary for practical applications, but due to their relatively high work functions, high operating voltages and high electrode temperatures are required. Such working conditions increase the risk of surface damage and unstable operation of the device. Thus, an improved field emitter device and any process which reduces the complexity of fabricating field emitters is clearly useful. In order to overcome the aforementioned problems, an important research activity was undertaken towards development of materials and coatings suitable for cold cathodes capable of performing the emission of high current density under relatively weak field. As described hereinbelow, the present inventors have found a solution thereto.
The present invention can be better appreciated with an understanding of the related physics. General electric emission can be analogized to the ionization of a free atom. Prior to ionization, the energy of the electrons in an atom is lower than electrons at rest in a vacuum. In order to ionize the atom, energy must be supplied to the electrons in the atom. That is, the atom fails to spontaneously emit electrons unless the electrons are provided with energy greater than or equal to the electrons at rest in the vacuum. Energy can be provided by numerous means, such as by heat or irradiation with light. When sufficient energy is imparted to the atom, ionization occurs, and the atoms release one or more electrons.
Several type of electron emissions are known. One such type is field emission. Field emission refers to the emission of electrons due to an electric field.
In field emission (or cold emission), electrons under the influence of a strong electric field are liberated out of a substance (usually a metal or semiconductor) into a dielectric (usually a vacuum). The electrons tunnel through a potential barrier. Field emission is therefore a quantum-mechanics phenomenon. A more detailed discussion of the physics of field emission can be found in U.S. Pat. No. 4,663,559 to Christensen.
The shape of a field emitter effects its emission characteristics. Field emission is most easily obtained from sharply pointed needles or tips whose ends have been smoothed into a nearly hemispherical shape by heating. Tip radii as small as 100 nanometers have been reported. As an electric field is applied, the electric lines of force diverge radially from the tip and the emitted electron trajectories initially follow these lines of force. Fabrication of such fine tips, however, normally requires extensive fabrication facilities to finely tailor the emitter into a conical shape. Further, it is difficult to build large area field emitters since the cone size is limited to the lithographic equipment. It is also difficult to perform fine feature lithography on large area substrates as required by flat panel display type applications. Thus, there is a need for a method of making field emitters which do not suffer from these problems.
The electron affinity (equal to work function) of the electron emitting surface or tip of a field emitter also effects emission characteristics. Electron affinity thus corresponds to the voltage (or energy) required to extract or emit electrons from a surface. The lower the electron affinity, the lower the voltage required to produce a particular amount of emission. If the electron affinity is negative, then the surface shall spontaneously emit electrons until stopped by space charge, although the space charge can be overcome by applying a small voltage, e.g., 5 volts. Compared to the 10,000 to 20,000 volts normally required to achieve filed emission from tungsten, a widely used field emitter, such small voltages are highly advantageous. There are several materials which exhibit negative electron affinity, but almost all of these materials are alkali metal based. Alkali metals are quite sensitive to atmospheric conditions and tend to decompose when exposed to air or moisture. Additionally, alkali metals have low melting points, typically below 1 00° C., which may be unsuitable in certain applications.
Thus, there became a need for other materials or coatings to be used in field emitters.
One such material that has been suggested is carbon nanotubes.
Two kinds of carbon nanotubes are currently available: multi-walled nanotubes (MWNTS) and single-walled nanotubes (SWNTS). MWNTS are comprised of 2 to 30 concentric graphitic layers, have outer diameters from 10 to 50 nm and their length is greater than 10 μm. SWNTS are much thinner: 1.0 to 1.4 nm in diameter, while their length is 100 um.
Carbon nanotubes have attracted considerable attention as a promising material for electron emitters cathodes for cathode nanotubes and other similar devices.
For example, field emission from an isolated multi-walled nanotube was first reported by Rinzler, AG, et al. in Science, 1995, 269, 1550-3. Subsequently, many experimental results were published on field emission for multi-walled nanotubes, such as by Collins, et al., in App. Phys. Letter, 1996, 96, 1969-70; Saito, et al., in Nature, 1997, 389, 554-5; Bonard, et al., in Ultramicroscopy, 1998, 73, 7-15 and for single walled nanotubes, such as by Saito, et al, in Jpn. J. Appl. Phys, 1997, 36, L1340-2, and Bonard, et al., in App. Phys. Lett., 1998, 73, 918-20. Very recently, nanotubes have been applied as cold electron sources in display devices as described by Saito, et al. in Jpn. J. Appl. Phys., 1998, 37, L346-8 and they have successfully manufactured nanotube based cathode ray tube (CRT) lighting elements.
Field emission has also been observed in these two kinds of nanotube carbon structures. L. A. Chemozatonskii, et al., in Chem. Phys. Letters, 233 63 (1995) and Mat. Res. Soc. Symp. Proc., Vol. 359, 99 (1995) have produced films of nanotube carbon structures on various substrates by the electron evaporation of graphite in 10−5-106 Torr. These films consist of aligned tube-like carbon molecules standing next to one another. Two types of tube-like molecules are formed; the Atubelites whose structure includes single-layer graphite-like tubules forming filaments-bundles 10-30 nm in diameter and the B-tubelites, including mostly multiplayer graphite-like tubes 10-30 nm in diameter with conoid or dome-like caps. The authors report considerable field electron emission from the surface of these structures and attribute it to the high concentration of the field at the nanodimensional tips. B. H. Fishbine, et al., Mat. Res. Soc. Symp. Proc., Vol. 359, 93 (1995) discuss experiments and theory directed towards the development of a buckytube (i.e., a carbon nanotube) cold field emitter array cathode.
The carbon nanotubes possess the following properties favorable for field emitters: (1) favorable electronic structure, (2) good in plane electrical conductivity, (3) a sharp tip, (4) high chemical stability and (5) high mechanical strengths.
Although the present inventors have found that cathodes made of nanotubes, such as multi-walled carbon nanotubes, have shown good electron field emission characteristics, the present inventors have noted traces of evaporation of cathode material, i.e., carbon from the nanotubes, on the walls of the device during electron field emission, especially in the case of local overheating of the emitting surface in high vacuum that usually exists within devices based on this effect, such as CRTs, flat panel displays and the like. Thus, the present inventors searched for a means to prevent the evaporation of carbon cathode material as well as improve the electron emission characteristics of the cathodes made of carbon nanotubes.
The present inventors felt that this could be achieved by coating the nanotubes. One of the roles of the coating is to lower the effective work function for the electrons escaping from the cathode and to increase, as a consequence thereof, the intensity of the emission. In order to fulfill that role, the coating material should have either a negative or very small positive electron affinity. Since the effective work function is proportional to the dielectric constant of the coating and to the difference between the conduction band minimum and the Fermi level, these two parameters should be minimized by modifying morphological or physio-chemical properties of the coating. The coating should also provide the passive protection for the surface of the sharp emitter by shielding it against ion bombardment and protecting it from over heating. Hardness and good thermal conductivity are, thus, desirable properties for the coating material. Moreover, the coating material should also substantially reduce or prevent the evaporation of carbon from the nanotubes during the operation of the electron field emitter comprised of nanotubes.
Diamond has emerged a promising material due to its chemical inertness and stability and favorable combination of thermal and electronic properties.
However, for a full understanding about diamonds, again a slight digression is necessary.
It has been experimentally shown that the (111) surface of diamond crystal has an electron affinity of −0.7 to −0.5 electron volts, showing it to possess negative electron affinity. It has been alleged that diamond has favorable electron emission properties comprising low voltage operation, high current density, robust operation, low emission noise and high chemical inertness against emission surface contamination. Geis, et al., IEEE Electron Device Lett., 12, 465 (1991), Geis, et al., App. Phys. Lett., 67, 1328 (1995).
A common conception about diamonds is that they are very expensive to fabricate. This is not always the case, however. The use of plasma chemical vapor deposition processes appears to provide promising ways to bring down the cost of producing high quality diamond thin films. It should be noted that diamond thin films cost significantly less than the high quality diamonds used in jewelry. Moreover, high fidelity audio speakers with diamond thin films as vibrating cones are already commercially available.
Diamond cold cathodes have been reportedly used. The diamond cold cathodes are formed by fabricating mesa-etched diodes using carbon ion implantation into p-type diamond substrates. It has been alleged that the diamond can be doped either n- or p-type. In fact several methods show promise for fabricating n-type diamond, such as bombarding electron emission film with sodium, nitrogen or lithium during growth. However, in current practice, it is extremely difficult to fabricate n-type diamond, and efforts for n-type doping usually result in p-type diamond. Furthermore, p-type doping fails to take full advantage of the negative electron effect, and pure or undoped diamond is insulating and normally prevents emission.
It was initially assumed that the emission properties of the diamond originate exclusively in its negative electron affinity (NEA) permitting electrons to leave the diamond surface without (or with small) thermal activation (the vacuum electron energy level is lower than the conduction band of the diamond, and the energy barrier between them does not exist, or can be neglected as a result of the tunnel effect). Different deposition techniques have been used to form the coatings, and the obtained results have indicated that besides its negative electron affinity, some other factors such as textural features, microcompositional map and surface resistivity, can significantly influence the emission properties of the cathode. See, for example, Zhimov, et al., J. Vac. Sci. Technol., A15, 1733 (1997); Zhou, et al., J. Electrochem. Soc., 144, L224 (1997); Pryor, App. Phys. Lett., 68, 1802 (1996); Li, et al., J. Phy. D.: App. Phys., 30, 2271 (1997); Klages, Ann. Phys., A 56, 513 (1993); Zhu, et al., J. Vac. Sci. Technol., B14, 2011(1996); Meyers, et al., J. Vac. Sci. Technol., B14, 2024 (1996); and Givargizov, et al., J. Vac. Sci. Technol., B14, 2030 (1996). The results obtained through such a diversified research work have shed more light on the role of the WBGM (Wide Band Gap Materials) coatings and helped to establish a set of general criteria useful for the future direction of research and development in this field as well as for the optimization of the fabrication of efficient electron sources.
However, today's diamond coated cathodes do not perform as it might be expected and have low emission efficiency. This is explained in terms of the influence of the texture of the deposited diamond: the coatings usually consist of micrometer-size crystallites having (100), (110), and (111) planes as the exposed surfaces. The electrons are not emitted from those flat surfaces but rather from microtips or edges of the faceted crystallites, see, Zhu, et al. J. Appl. Phys., 78, 2707 (1995). As a consequence, only 1 to 10% of the diamond surface contributes to the electron emission and emission sites are not uniformly distributed.
Recently obtained results indicate that the problem of the efficiency of the diamond coating cold cathodes might be more complicated, and that the simple microtextural features can not account for the observed behavior. Various factors must be taken into account, such as establishing the effective work functions for electron emission and the influence of defects, impurities and morphology of the diamond coatings. There are indications that the processes occurring at the interface between metal (or semiconductor) and the diamond influence the emission more than the processes of electron “evaporation” from the diamond surface. See, Zhimov, et al., J. Vac. Sci. Technol., A15, 1733 (1997), Zhimov, Jour De Phys. IV, Colloque C5, 6, 107 (1996), and Givargizov, et al., J. Vac. Sci. Technol., B14, 2030 (1996). It is believed that the presence of different kinds of defects is accompanied by the widening of the typical diamond peak in Raman spectra (1332 cm−1) from FWHM<5 cm−1 up to FWHM˜11 cm−1, which is very close to the value for diamondlike-carbon (DLC) structures. Cathodes coated with ball-like and cauliflower-like diamond, having much smaller surface resistance than continuous high quality diamond, show, e.g., better emission properties. See, Li, et al., J. Phys. D.: App. Phys., 30, 2271(1997). Again, the improvement of the cathode characteristics is accompanied by deterioration of the diamond peak in the Raman spectrum and with simultaneous appearance of wide peaks typical for graphite phase (1568 cm−1) and for amorphous carbon (1482 cm−1). The fact that the thickness of the coating also influences the electron emission efficiency (see, Zhimou, et al, J. Vac. Sci. Technol. A15, 1733 (1999)) suggests that the electron transport through the coating should also be taken into account.
Moreover, no one heretofore had used diamonds or diamond like carbons to coat nanotubes. The typical methods used in the art for coating a substrate with diamonds utilized chemical vapor deposition, sputtering and ion beam deposition using as raw materials methane and hydrogen, and these produced diamond films of having a thickness in the range of about 500 to 5000 angstroms, which dimensions are too large to coat the nanotubes used in field electron emitters.
Heretofore, it was unknown whether coating the nanotubes with diamonds whose dimensions are decreased by ten fold would be successful in enhancing the electron field emission characteristics of the nanotubes. Moreover, it was unknown heretofore whether diamond coatings would be capable of preventing the evaporation of cathode material, e.g., carbon nanotubes during electron field emission.
A general picture of the mechanisms involved in electron emission encompasses the tunneling of electrons from metal into conduction band of the coating. Under influence of the field, electrons are transported through the conduction band to the surface where they escape into vacuum by tunneling. The emission current is calculated by the one-dimensional Fowler-Nordheim equation. See, e.g., Fowler, et al. Proc. Roy. Soc. London, A19, 173 (1928). This equation was adapted and applied to various emitting surfaces. In the majority of the studied cases, the Fowler-Nordheim diagrams are linear, suggesting that the tunneling step is the controlling one for the whole process of electron emission. This general picture often accounts for the field electron emission, but the mechanisms of each of the steps involved in the process are still elusive, and several models have been employed in an attempt to account for the experimental data. See, e.g., Zhirnov, et al, M. R. S. Bulletin, 23 42 (1998).
From the foregoing, it is apparent there is a clear need for a thermodynamically stable material with electron affinity for use as coating over a cathode comprised of nanotubes. The present inventors have found that coating the nanotube particles of the field emitters with either diamond or diamond like carbon has solved the problems described hereinabove. More specifically, they found that diamond and diamond like carbon coating on nanotubes have not only enhanced the electron emission characteristics thereof, but also have retarded and/or prevented the evaporation of carbon from carbon nanotubes during the operation of an electron field emitter having a cathode comprised of nanotubes.